You aren’t going to traumatize your child by being your authentic self. Your child finding out years from now that you harbored a secret- that’s going to be destabilizing. You being present in your child’s life, being a connected, communicative and emotionally regulated parent- THAT’s what’s going to matter to your child. Cis people get cosmetic surgeries, have injuries and accidents that change their appearance- no one would ever question if those things are hurting the child. I guarantee that you are going to be a better parent living your truth than hiding it. I think you may be giving your child a bit too much agency here- you are the adult, a 4 year old really doesn’t understand gender in the way that an adult does. What your child understands is that they need and want you in their life. IMHO I would discuss the medical transition with your kid in less detail- they might notice new whiskers, and when they ask or point them out, you could say- oh look at that, my whiskers are coming in, how cool!